This invention relates to an image processing method and apparatus and, more particularly to an image processing method and apparatus for adding a specific pattern on an input image signal to form an output image.
Recently, color printers have become popular and been utilized as various representation means for users. Especially, a color page printer using electrophotographic method attracts public attention by its quiet operation, high image quality and high-speed printing.
One color page printer, a full-color laser-beam printer performs multi-color image formation and recording by generally-known four-step printing. At the first step, a laser beam is scanned on an electrostatic drum in a main-scanning direction, to make a first development using a first toner, and the developed latent image is transferred to a recording medium such as a recording sheet. Similarly, at the second to fourth steps, second to fourth toners are used to develop second to fourth latent images.
In these four steps, Y (yellow), M (magenta), C (cyan) and K (black) toners are used for image formation, and the four latent images are transferred onto the recording medium, thus a color image is obtained.
Next, the recording method of this conventional full-color printer will be described with reference to FIGS. 14 and 15. FIG. 14 is a cross-sectional view showing the construction of the conventional full-color printer. FIG. 15 is a block diagram showing the flows of various signals used by the printer in FIG. 14.
In FIG. 14, an electrostatic drum 201, which rotates at a fixed speed in a direction represented by the arrow, is charged by an electrostatic charger 204 to a predetermined voltage and a predetermined polarity. Next, a recording sheet P is supplied from a paper cassette 215 by a paper feeding roller 214, at predetermined timing, one sheet at a time. When a detector 202 detects the front end of the recording sheet P, a semiconductor laser 205 emits laser light L, modulated by an image signal VDO (8-bit/pixel/color), to a polygon mirror 207 driven by a scanner motor 206. The laser light L is reflected by the polygon mirror 207, then guided onto the electrostatic drum 201 via a lens 208 and a mirror 209, and sweeps on the electrostatic drum 201. On the other hand, a signal from the detector 202 (hereinafter referred to as "TOPSNS") is outputted as a vertical synchronizing signal to an image forming unit 250 as shown in FIG. 15. When a detector 217 detects the laser light L, it outputs a beam-detect signal (hereinafter abbreviated to as "BD signal"), which is a horizontal synchronizing signal, to the image forming unit 250. Then, the image signal VDO is sequentially transferred to the semiconductor laser 205 in synchronization with the BD signal.
The scanner motor 206 rotates at a fixed speed, in accordance with a signal S2 from a frequency divider 221 which divides a signal S1 from a reference oscillator 220, under the control of a motor controller 225.
The electrostatic drum 201 is scan-exposed in synchronization with the BD signal, then a first electrostatic latent image is developed by a developer 203Y having yellow toner, and a yellow toner image is formed on the electrostatic drum 201.
On the other hand, immediately before the front end of the recording sheet P reaches a transfer-start position, a predetermined transfer bias voltage of an opposite polarity to that of the toner is applied to the transfer drum 216. The yellow toner image is transferred onto the recording sheet P, and at the same time, the recording sheet P is electrostatically attached to the surface of the transfer drum 216.
Next, a second electrostatic latent image is formed on the electrostatic drum 201 by the scanning of the laser light L, then the second latent image is developed by a developer 203M having magenta toner. The position of the magenta toner image on the electrostatic drum 201 is aligned, by the TOPSNS signal, with the position of the first (yellow) toner image, and the second toner image is transferred onto the recording sheet P.
In a similar manner, a third electrostatic latent image is developed by a developer 203C having cyan toner, then the position of the cyan toner image is aligned with that of the previous magenta image, and the third toner image is transferred onto the recording sheet P. Finally, a fourth electrostatic latent image is developed by a developer 203K having black toner, the position of the black toner image is aligned with that of the previous cyan image, and the fourth toner image is transferred onto the recording sheet P.
At each step, the VDO signal for one page is sequentially outputted to the semiconductor laser 205. After each transfer, untransferred toner on the electrostatic drum 201 is scraped off by a cleaner 210.
Thereafter, as the front end of the recording sheet P, on which the four toner images have been transferred, approaches a separation claw 212, the separation claw 212 moves to contact the surface of the transfer drum 216 so as to separate the recording sheet P from the transfer drum 216. The separation claw 212 is in contact with the transfer drum 216 till the rear end of the recording sheet P is separated from the transfer drum 216, thereafter, returns to the initial position. An electrostatic discharger 211 removes the accumulated charge on the recording sheet P, thus assists separation of the recording sheet P by the separation claw 212; and reduces atmospheric discharge upon separation of recording sheet.
Finally, the developed color image on the recording sheet P is fixed by a fixing roller 213 and the recording sheet P is discharged to a tray 229.
Note that the image forming unit 250 in FIG. 15 is a generic term of all the elements in FIG. 14 excluding the semiconductor laser 205, the scanner motor 206, the polygon mirror 207 and the detectors 202 and 217.
FIG. 16 is a timing chart showing the relation between the TOPSNS signal and the VDO signal. In FIG. 16, term A1 corresponds to the first-toner color printing; A2, printing of the second-toner color printing; A3, the third-toner color printing; and A4, the fourth-toner color printing. The color printing for one page is from the term A1 to the term A4.
Next, image signal processing will be described.
FIG. 17 is a block diagram showing the functional construction of a conventional full-color printer 302. In FIG. 17, a host interface 303 receives print information 307 from an external device e.g. a host computer 301, transmits a control signal 308 included in the received print information 307 to a printer controller 304, and transmits an image signal 309 also included in the print information 307 to an image processor 305. The image processor 305 outputs a signal to drive a semiconductor laser 306. The printer controller 304 controls the image processor 305 by a control signal 310.
FIG. 18 is a block diagram showing the detailed construction of the image processor 305 shown in FIG. 17. In FIG. 18, a color processor 351 receives a 24-bit RGB image signal from the host interface 303 shown in FIG. 17, and sequentially converts the input RGB signal into a YMCK signal at predetermined timing. That is, the color processor 351 converts the input RGB signal, into an 8-bit VDO signal indicative of a Y signal on one occasion; into an 8-bit VDO signal indicative of a M signal on another occasion; into an 8-bit VDO signal indicative of a C signal on still another occasion; and into an 8-bit VDO signal indicative of a K signal on still another occasion.
FIG. 19 is a timing chart showing the color signal conversion by the color processor 351. In FIG. 19, terms A1 to A4 represent the respective toner color printing operations, as described in FIG. 16. The same color signal set (R1, G1 and B1 in FIG. 19) is used for respective four toner color printing operations. The color of each printing operation is indicated by the 2-bit color designation signal. In the color designation signal, numeral "B" added to the respective values indicates that the values are in binary representation.
The YMCKVDO signal from the color processor 351 is .gamma.-corrected by a y corrector 352, and outputted as an 8-bit signal, then inputted into a pulse-width modulator (hereinafter abbreviated to "PWM") 353. The PWM 353 latches the 8-bit image signal by a latch 354 in synchronization with a rising edge of an image clock (VCLK). Then the PWM 353 converts the latched digital data into corresponding analog voltage by a D/A converter 355, and inputs the analog voltage into an analog comparator 356.
On the other hand, the image clock (VCLK) is inputted into a triangular-wave generator 358, which converts the image clock into a triangular wave and outputs it to the analog comparator 356.
The analog comparator 356 compares the triangular wave from the triangular-wave generator 358 and the analog signal from the D/A converter 355, and outputs a pulse-width modulated signal.
FIG. 20 is a timing chart showing timings of various control signals related to the PWM signal generation process.
In FIG. 20, when the 8-bit image data inputted into the PWM 353 has a maximum value "FF(H)", a PWM signal of the highest level (maximum width) is outputted, while when the input 8-bit data has a minimum value "00(H)", a PWM signal of the lowest level (minimum width) is outputted.
The improved printer performance as described above enables high-quality printing, however, it poses a problem that such printers are used for criminal acts such as forgery of bank notes and securities. It is conceivable that as printers are further developed with higher image quality, this type of crimes may increase.
One method to prevent forgery is to add a coded manufactured serial number of printer onto every output from the printer. This enables to easily find the printer used in illegal printing from forged bank notes and securities, thus assists in criminal investigation.
However, in a printer having a triangular-wave generator, capable of generating a triangular wave having a plurality of frequencies (or periods) for image formation at a relatively high resolution and for image formation at a relatively low resolution, especially upon image formation at the high resolution, as a triangular wave of a high frequency (or short period) is generated, it is difficult to represent a coded manufactured serial number of the printer on formed images at a uniform density or an even density. That is, as recent technical improvement attains higher resolution image formation, overlaying of coded information becomes difficult because toner cannot be properly transferred and fixed onto paper. Unless code formation is constantly made, to interpret a printed code to define a printer used for image formation of printed matter is unreliable.